1. Field of the Invention
The present invention relates to a structure for connecting a non-radiative dielectric waveguide and a metal waveguide, which is incorporated in, for example, a millimeter wave integrated circuit and used for the transmission of high frequency signals, and capable of transmitting and receiving high-frequency signals inform of radiowaves. The invention also relates to a millimeter wave transmitting/receiving module and a millimeter wave transmitter/receiver.
2. Description of the Related Art
Conventionally, non-radiative dielectric waveguides (also referred to as “NRD guides” in the following), in which a dielectric strip is sandwiched between a pair of parallel planar conductors, are known as one type of transmission line for high-frequency signals. When an NRD guide is incorporated on a printed circuit board or the like, the circuit has to be designed such that the NRD guide can be connected to other high-frequency transmission lines, antennas, etc., in which case it is important to design the connection such that the deterioration of the transmission characteristics is kept small.
As an alternative structure for connecting a high-frequency transmission line, a structure for connecting an NRD guide and a micro-strip line has been proposed. A general structure thereof is shown in FIG. 19. In the NRD guide shown in FIG. 19, a dielectric strip 3 is arranged between a pair of parallel planar conductors 11, 12. A slot hole 13 is formed in the parallel planar conductor 11, and a dielectric substrate 14 on which a central conductor 15 is formed is placed on the surface including the slot hole 13 in the parallel planar conductor 11, such that the slot hole 13 is arranged in a predetermined positional relation with respect to a terminal end of the central conductor 15, whereby the NRD guide can be connected electromagnetically to a microstrip line through the slot hole 13.
In another configuration for connecting the dielectric strip of an NRD guide to a metal waveguide (not shown in the drawings), an output end or an input end of the dielectric strip is taper-shaped, and one end of a rectangular horn-shaped metal waveguide is placed near that tapered portion.
As another structure for connecting an NRD guide to a metal waveguide, it has been proposed to provide an aperture in a portion of the parallel planar waveguides corresponding to the dielectric strip, and to connect this aperture to the open end of the metal waveguide (see Japanese Unexamined Patent Publication JP-A 2000-22407).
However, when the dielectric strip of an NRD guide is provided with a taper-shaped end, as described above, to connect the dielectric strip to a metal waveguide, the length of this tapered portion has to be at least twice the wavelength of the high-frequency signal, so that there is the drawback that it is not suitable for the miniaturization of a millimeter wave integrated circuit.
On the other hand, the configuration shown in FIG. 19 is suitable for miniaturization, but for high-frequency signals in the millimeter waveband of at least 30 GHz, the transmission loss when using a microstrip line is large, so that this conventional connection structure is not suited for circuit boards in which the signal frequency is 30 GHz or higher.
It is known that a metal waveguide can be used instead of a microstrip line as a propagation structure with low transmission loss, like an NRD guide, in the millimeter wave band above 30 GHz, and it is also important to use metal waveguides in circuit design. In one example, an aperture is provided in the portion of the parallel planar conductor that corresponds to the dielectric strip, and this aperture is connected with the open terminal end of the dielectric strip (see JP-A 2000-22407). However, with this configuration, signals tend to be reflected and leak at the portion connecting the dielectric strip and the portion of the portion of the parallel planar conductor corresponding to the dielectric strip, and this structure is not satisfactory with respect to keeping signal losses small.